Reduction of the extrinsic base-collector capacitance (Cbex) improves the heterojunction bipolar transistor's (HBT's) speed. Both ft (cutoff frequency) and fmax (maximum oscillation frequency) are dependent on Cbox. In conventional HBTs much of the extrinsic base-collector junction area is required for forming the base contacts. Additionally, the metal contacts (e.g., vias) to the extrinsic base are not self-aligned to the emitter, but defined by lithography. This further increases the extrinsic base area since the layout design rules must allow for overlay error.
It is therefore desirable to fabricate an HBT having a base self-aligned to the emitter, and a small (preferably sub-lithographic; less than 100 nm) extrinsic base region to which metal contacts are also made self-aligned. The extrinsic base resistance can also be minimized by thickening the extrinsic base and by converting the top portion of the extrinsic base into a metal-semiconductor alloy such as, for example, a silicide.
In view of the foregoing, there is a need for providing an HBT with a small base-collector capacitance and a low extrinsic base resistance. The present invention provides an HBT with a sub-lithographic extrinsic base that is defined by the footprint of a sidewall spacer made adjacent to the emitter. The fabrication of the extrinsic base is therefore self-aligned to the emitter. To lower the base resistance, the extrinsic portion of the base is first thickened by selective epitaxy and then the top portion of the extrinsic base is converted into a metal-semiconductor alloy. The metal contacts to the extrinsic base are also fabricated as part of a silicidation process, and are self-aligned to the extrinsic base.